muscari: switch bootloader to systemd-boot (UEFI)

Changes:
- boot.loader: generic-extlinux-compatible (BIOS) → systemd-boot (UEFI)
- YC standard-v3 VM is UEFI by default; systemd-boot is the modern
  choice for single-disk cloud VMs (no GRUB complexity)
- Comments updated: nixos-anywhere → nixos-install (in tmux), aligning
  with the documented install procedure in projects/servers/muscari/SETUP.md
- .gitignore: add hosts/*/hardware-configuration.nix as defense-in-depth
  (muscari uses disko and has no hw-config file; this guards future hosts
  from leaking UUIDs/MACs to the public forgejo repo)

Context:
- muscari VM specs: 2 vCPU / 8 GB / 100 GB SSD (was incorrectly documented
  as 4/16 in muscari/README.md; fixed there too)
- Flake builds cleanly:
    nix flake show           → nixosConfigurations.muscari
    hostname                 → "muscari"
    systemd-boot.enable      → true
    fileSystems."/"         → ext4 (disko-derived)
    fileSystems."/boot"     → vfat (disko-derived)

Verified: nix --extra-experimental-features 'nix-command flakes' flake show
and eval .#nixosConfigurations.muscari.config.{networking.hostName,
boot.loader.systemd-boot.enable, fileSystems} all pass.

Session: verify plan

.gitignore

@@ -21,4 +21,8 @@ devenv.yaml
 # age key (НИКОГДА не коммитить!)
 *.age-key
 *age-keys.txt
-*age-keys.json
+*age-keys.json
+
+# hardware-configuration per-host (защита от случайного коммита в публичный repo)
+# muscari использует disko и не имеет этого файла — это defense-in-depth для будущих хостов
+hosts/*/hardware-configuration.nix